Static and dynamic properties of hydrogen sulphide in hydroquinone clathrates

A variety of investigations has been undertaken on hydrogen sulphide hydroquinone clathrates, with the aim of achieving a better understanding of the motions of the guest molecules and of the phase transition near 8 K, Measurements have been made, over a wide temperature range including the phase transition, of heat capacities on H2S, D2S, and HDS clathrates, dielectric constants perpendicular and parallel to the trigonal axis of single crystal specimens, incoherent quasi-elastic neutron scattering, and neutron diffraction. All of the data are consistent with the H2S molecular dipole being able to rotate rather easily about an axis parallel to the trigonal crystal c axis. Rotations about the other axes are much more difficult. The rotation about the trigonal axis is almost free at high temperatures but there is evidence for three preferred orientations at temperatures just above the phase transition. No discrete energy states are observed at any temperature due to strong guest–cage and guest–guest interactions. The experiments provide direct evidence that the phase transition is associated with a cooperative freezing out of the dipole reorientations.